What is a high purity ball valve?The High Purity Ball Valve is a flow control device that meets industry standards for material and design purity.Valves in the high-purity process are used in two major fields of application:
These are used in “support systems” such as processing cleaning steam for cleaning and temperature control.In the pharmaceutical industry, ball valves are never used in applications or processes that may come into direct contact with the end product.
What is the industry standard for high purity valves?The pharmaceutical industry derives valve selection criteria from two sources:
ASME/BPE-1997 is an evolving normative document covering the design and use of equipment in the pharmaceutical industry.This standard is intended for the design, materials, construction, inspection and testing of vessels, piping and related accessories such as pumps, valves and fittings used in the biopharmaceutical industry.Essentially, the document states, “…all components that come into contact with a product, raw material or product intermediate during manufacturing, process development or scale-up…and are a critical part of product manufacturing, such as water for injection (WFI), Clean steam, ultrafiltration, intermediate product storage and centrifuges.”
Today, the industry relies on ASME/BPE-1997 to determine ball valve designs for non-product contact applications.The key areas covered by the specification are:
Valves commonly used in biopharmaceutical process systems include ball valves, diaphragm valves, and check valves.This engineering document will be limited to a discussion of ball valves.
Validation is a regulatory process designed to ensure the reproducibility of a processed product or formulation.The program indicates to measure and monitor mechanical process components, formulation time, temperature, pressure and other conditions.Once a system and the products of that system are proven to be repeatable, all components and conditions are considered validated.No changes may be made to the final “package” (process systems and procedures) without revalidation.
There are also issues related to material verification.An MTR (Material Test Report) is a statement from a casting manufacturer that documents the casting’s composition and verifies that it came from a specific run in the casting process.This level of traceability is desirable in all critical plumbing component installations across many industries.All valves supplied for pharmaceutical applications must have MTR attached.
Seat material manufacturers provide composition reports to ensure seat compliance with FDA guidelines.(FDA/USP Class VI) Acceptable seat materials include PTFE, RTFE, Kel-F and TFM.
Ultra High Purity (UHP) is a term intended to emphasize the need for extremely high purity.This is a term widely used in the semiconductor market where the absolute minimum number of particles in the flow stream is required.Valves, piping, filters, and many materials used in their construction typically meet this UHP level when prepared, packaged, and handled under specific conditions.
The semiconductor industry derives valve design specifications from a compilation of information managed by the SemaSpec group.The production of microchip wafers requires extremely strict adherence to standards to eliminate or minimize contamination from particles, outgassing and moisture.
The SemaSpec standard details the source of particle generation, particle size, gas source (via soft valve assembly), helium leak testing, and moisture inside and outside the valve boundary.
Ball valves are well proven in the toughest applications.Some of the key benefits of this design include:
Mechanical Polishing – Polished surfaces, welds and surfaces in use have different surface characteristics when viewed under a magnifying glass.Mechanical polishing reduces all surface ridges, pits and variances to a uniform roughness.
Mechanical polishing is done on rotating equipment using alumina abrasives.Mechanical polishing can be achieved by hand tools for large surface areas, such as reactors and vessels in place, or by automatic reciprocators for pipes or tubular parts.A series of grit polishes are applied in successive finer sequences until the desired finish or surface roughness is achieved.
Electropolishing is the removal of microscopic irregularities from metal surfaces by electrochemical methods.It results in a general flatness or smoothness of the surface that, when viewed under a magnifying glass, appears almost featureless.
Stainless steel is naturally resistant to corrosion due to its high chromium content (usually 16% or more in stainless steel).Electropolishing enhances this natural resistance because the process dissolves more iron (Fe) than chromium (Cr).This leaves higher levels of chromium on the stainless steel surface.(passivation)
The result of any polishing procedure is the creation of a “smooth” surface defined as average roughness (Ra).According to ASME/BPE; “All polishes shall be expressed in Ra, microinches (m-in), or micrometers (mm).”
Surface smoothness is generally measured with a profilometer, an automatic instrument with a stylus-style reciprocating arm.The stylus is passed through the metal surface to measure peak heights and valley depths.The average peak heights and valley depths are then expressed as roughness averages, expressed in millionths of an inch or microinches, commonly referred to as Ra.
The relationship between the polished and polished surface, the number of abrasive grains and the surface roughness (before and after electropolishing) is shown in the table below.(For ASME/BPE derivation, see Table SF-6 in this document)
Micrometers are a common European standard, and the metric system is equivalent to microinches.One microinch is equal to about 40 micrometers.Example: A finish specified as 0.4 microns Ra is equal to 16 micro inches Ra.
Due to the inherent flexibility of ball valve design, it is readily available in a variety of seat, seal and body materials.Therefore, ball valves are produced to handle the following fluids:
The biopharmaceutical industry prefers to install “sealed systems” whenever possible.Extended Tube Outside Diameter (ETO) connections are in-line welded to eliminate contamination outside the valve/pipe boundary and add stiffness to the piping system.Tri-Clamp (hygienic clamp connection) ends add flexibility to the system and can be installed without soldering.Using Tri-Clamp tips, piping systems can be more easily disassembled and reconfigured.
Cherry-Burrell fittings under the brand names “I-Line”, “S-Line” or “Q-Line” are also available for high purity systems such as the food/beverage industry.
Extended Tube Outside Diameter (ETO) ends allow in-line welding of the valve into the piping system.ETO ends are sized to match the pipe (pipe) system diameter and wall thickness.The extended tube length accommodates orbital weld heads and provides sufficient length to prevent damage to the valve body seal due to welding heat.
Ball valves are widely used in process applications because of their inherent versatility.Diaphragm valves have limited temperature and pressure service and do not meet all standards for industrial valves.Ball valves can be used for:
Additionally, the ball valve center section is removable to allow access to the internal weld bead, which can then be cleaned and/or polished.
Drainage is important to keep bioprocessing systems in clean and sterile conditions.The liquid remaining after draining becomes a colonization site for bacteria or other microorganisms, creating an unacceptable bioburden on the system.Sites where fluid builds up can also become corrosion initiation sites, adding additional contamination to the system.The design portion of the ASME/BPE standard requires design to minimize hold-up, or the amount of liquid that remains in the system after draining is complete.
A dead space in a piping system is defined as a groove, tee, or extension from the main pipe run that exceeds the amount of pipe diameter (L) defined in the main pipe ID (D).A dead space is undesirable because it provides a entrapment area that may not be accessible through cleaning or sanitizing procedures, resulting in product contamination.For bioprocessing piping systems, a 2:1 L/D ratio can be achieved with most valve and piping configurations.
Fire dampers are designed to prevent the spread of flammable liquids in the event of a process line fire.The design uses a metal back seat and anti-static to prevent ignition.The biopharmaceutical and cosmetic industries generally prefer fire dampers in alcohol delivery systems.
FDA-USP23, Class VI approved ball valve seat materials include: PTFE, RTFE, Kel-F, PEEK and TFM.
TFM is a chemically modified PTFE that bridges the gap between traditional PTFE and melt-processable PFA.TFM is classified as PTFE according to ASTM D 4894 and ISO Draft WDT 539-1.5.Compared to traditional PTFE, TFM has the following enhanced properties:
Cavity-filled seats are designed to prevent buildup of materials that, when trapped between the ball and the body cavity, could solidify or otherwise hinder the smooth operation of the valve closing member.High-purity ball valves used in steam service should not use this optional seat arrangement, as steam can find its way under the seat surface and become an area for bacterial growth.Because of this larger seating area, cavity-filler seats are difficult to properly sanitize without dismantling.
Ball valves belong to the general category of “rotary valves”.For automatic operation, two types of actuators are available: pneumatic and electric.Pneumatic actuators utilize a piston or diaphragm connected to a rotating mechanism such as a rack and pinion arrangement to provide rotational output torque.Electric actuators are basically gear motors and are available in a variety of voltages and options to suit ball valves.For more information on this topic, see “How to Select a Ball Valve Actuator” later in this manual.
High Purity Ball Valves can be cleaned and packaged to BPE or Semiconductor (SemaSpec) requirements.
Basic cleaning is performed using an ultrasonic cleaning system that uses an approved alkaline reagent for cold cleaning and degreasing, with a residue-free formula.
Pressure-containing parts are marked with a heat number and are accompanied by an appropriate certificate of analysis.A Mill Test Report (MTR) is recorded for each size and heat number.These documents include:
Sometimes process engineers need to choose between pneumatic or electric valves for process control systems.Both types of actuators have advantages and it is valuable to have the data available to make the best choice.
The first task in choosing the type of actuator (pneumatic or electric) is to determine the most efficient power source for the actuator.The main points to consider are:
The most practical pneumatic actuators use an air pressure supply of 40 to 120 psi (3 to 8 bar).Typically, they are sized for supply pressures of 60 to 80 psi (4 to 6 bar).Higher air pressures are often difficult to guarantee, while lower air pressures require very large diameter pistons or diaphragms to generate the required torque.
Electric actuators are typically used with 110 VAC power, but can be used with a variety of AC and DC motors, both single and three-phase.
temperature range.Both pneumatic and electric actuators can be used over a wide temperature range.The standard temperature range for pneumatic actuators is -4 to 1740F (-20 to 800C), but can be extended to -40 to 2500F (-40 to 1210C) with optional seals, bearings and greases.If control accessories (limit switches, solenoid valves, etc.) are used, they may be temperature rated differently than the actuator, and this should be taken into account in all applications.In low temperature applications, air supply quality in relation to dew point should be considered.Dew point is the temperature at which condensation occurs in the air.Condensation can freeze and block the air supply line, preventing the actuator from operating.
Electric actuators have a temperature range of -40 to 1500F (-40 to 650C).When used outdoors, the electric actuator should be isolated from the environment to prevent moisture from entering the inner workings.If condensation is drawn from the power conduit, condensation may still form inside, which may have collected rainwater prior to installation.Also, because the motor heats the inside of the actuator housing when it is running and cools it when it is not running, temperature fluctuations can cause the environment to “breathe” and condense.Therefore, all electric actuators for outdoor use should be equipped with a heater.
It is sometimes difficult to justify the use of electric actuators in hazardous environments, but if compressed air or pneumatic actuators cannot provide the required operating characteristics, electric actuators with appropriately classified housings can be used.
The National Electrical Manufacturers Association (NEMA) has established guidelines for the construction and installation of electric actuators (and other electrical equipment) for use in hazardous areas.The NEMA VII guidelines are as follows:
VII Hazardous Location Class I (Explosive Gas or Vapor) Meets the National Electrical Code for applications; meets the specifications of Underwriters’ Laboratories, Inc. for use with gasoline, hexane, naphtha, benzene, butane, propane, acetone, Atmospheres of benzene, lacquer solvent vapors and natural gas.
Almost all electric actuator manufacturers have the option of a NEMA VII compliant version of their standard product line.
On the other hand, pneumatic actuators are inherently explosion-proof.When electrical controls are used with pneumatic actuators in hazardous areas, they are often more cost-effective than electric actuators.The solenoid-operated pilot valve can be installed in a non-hazardous area and piped to the actuator.Limit switches – for position indication – can be installed in NEMA VII enclosures.The inherent safety of pneumatic actuators in hazardous areas makes them a practical choice in these applications.
Spring returns.Another safety accessory that is widely used in valve actuators in the process industry is the spring return (fail safe) option.In the event of a power or signal failure, the spring return actuator drives the valve to a predetermined safe position.This is a practical and inexpensive option for pneumatic actuators, and a big reason why pneumatic actuators are widely used throughout the industry.
If a spring cannot be used due to actuator size or weight, or if a double acting unit has been installed, an accumulator tank can be installed to store air pressure.